1. Technical Field
The present invention generally relates to solid state light emitting devices and, particularly, to a nitride-based semiconductor light emitting device with relatively low cost and high light extraction efficiency and methods for fabricating the same.
2. Description of Related Art
Nowadays, nitride-based semiconductor light emitting devices such as gallium nitride LEDs (i.e., light emitting diodes) have the advantages of low-power consumption and long life-span, etc, and thus are widely used for display, backlight, outdoor illumination, automobile illumination, etc. However, in order to achieve high luminous brightness, an improvement of light extraction efficiency of the conventional nitride-based LEDs is required.
Kao et al. have published a paper on IEEE photonics technology letters, vol. 19, No. 11, page 849-851 (June, 2007) entitled “light-output enhancement of nano-roughened GaN laser lift-off light-emitting diodes formed by ICP dry etching”, the disclosure of which is fully incorporated herein by reference. Kao et al. have proposed an approach for the improvement of the light extraction efficiency of the GaN LED, by way of roughening a light-emitting region of the GaN LED via an ICP-RIE (i.e., inductively coupled plasma-reactive ion etching) dry etching. In particular, a GaN-based layer structure is epitaxially grown on a c-face sapphire substrate, the GaN-based layer structure then is placed into a vacuum chamber which is fed with chlorine and argon for ICP-RIE dry etching. Consequently, a light-emitting region of the GaN-based layer structure is given a nano-roughened surface which facilitates the improvement of light extraction efficiency of the GaN LED.
However, the use of the c-face sapphire substrate would force the epitaxial growth of the GaN-based layer structure oriented along a c-axis <0001> crystal orientation. As a result, the surface atoms of the resultant GaN-based layer structure are entirely gallium metal atoms. Such configuration of the surface atoms results in the GaN-based layer structure exhibits an extremely strong polarity defect. Such polarity defect would cause the following issues: (1) a quantum well structure in the GaN-based layer structure which is oriented along the c-axis <0001> crystal orientation, would encounter a significantly strong quantum-confined stark effect (QCSE), so that an internal quantum efficiency of the GaN LED is lowered and thus the light extraction efficiency is degraded. (2) in order to roughen the surface of the light-emitting region, a low cost wet etching process is helpless, instead, a relatively high cost dry etching process with strong etching capability (e.g., ICP-RIE etching process) is absolutely needed. Unfortunately, the dry etching process is difficult to roughen sidewalls of the GaN-based layer structure for further improvement of light extraction efficiency of the GaN LED due to an inherent selective etching characteristic thereof.
Therefore, what is needed is a nitride-based semiconductor light emitting device with high extraction efficiency and relatively low cost and methods for fabricating the same.